[TowerTalk] Re: vert vs dipole

N4KG response below -
On Sun, 7 Dec 1997 08:00:00 EST Texas RF <TexasRF@aol.com> writes:
>Gary Myers inquired about modeling high dipoles vs vertical antennas,
>noting the dipole shows higher gain.
>
>LB where are you?? My recollection is that horizontal polarity gets
>all the benefits of ground reflection gain, 5db or so being typical of
the 6db
>theoretical maximum. Vertical polarization gets none of this benefit
>but I'll be danged if I can explain why. Perhaps one of you scientists
out
>there will enlighten us on this subject!
>
>73 de Gerald Williamson, K5GW
..............................................................
The ARRL Antenna Book devotes an entire chapter to Ground Effects.
(Chapter 3 in the 18th edition.)
Basically, the ground reflection coefficients are DIFFERENT for
horizontal
and vertical polarization. Not only that, but they differ with
frequency, wave
angle, and ground conductivity and dielectric constant.
For vertical polarization, the reflection coefficients drop off much
faster than
for horizontal polarization. In addition, the ground reflection
coefficients for
vertically polarization goes through a minimum at an angle known as the
Pseudo Brewster Angle. A plot of vertical reflection coefficients at 21
MHz
is given in Figure 7. Figure 8 shows how PBA varies with frequency and
ground conductivity.
Bottom line: Horizontal polarization is better IF the antenna is more
than
a half wavelength above ground. This height requirement becomes more
difficult at lower frequencies and therefore verticals enjoy more
popularity
and better relative performance on the low bands, especially when
surrounded by high conductivity soil or saltwater. (See 6Y4A results in
the 1997 CQ WW CW DX Contest. Outstanding results from parasitic
vertical antennas at the waters edge.)
de Tom N4KG
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